Resistance welding processes are widely used in the manufacture of sheet metal assemblies, such as automobile and aircraft bodies. Each resistance weld involves a sequence of electrical energy and mechanical pressure steps. The sequences are provided by a controller which governs the timing of the both the electrical and the mechanical steps. For example, the controller may establish a selected number of cycles of alternating current line power at a selected current, with a selected electrode force for each step of a welding operation. In carrying out the welding operation, the operator merely presses a button which sets the controller in operation. The controller then responds to an internal program to carry out all the steps necessary for the operation. The program of the controller is stored in an internal memory, and it may vary from a relatively few commands to a large number of commands, depending upon the simplicity or complexity of the welding operation.
A microprocessor controller for resistance welding machines is described, for example, in copending application Ser. No. 29,752 which was filed Apr. 13, 1979 in the name of the present inventor. The controller described in the copending application provides heat and pressure commands to the controlled resistance welding machine. The control system of the present invention finds application for use in conjunction with controllers of the type described in the copending application, since it is capable of introducing digital signals to the controller for controlling the operational cycles of the controller in accordance with the actual monitored condition of the weld being formed, rather than relying upon timed intervals. The control system of the invention senses the voltage across the silicon controlled rectifiers or ignitrons of the controller to detect the power factor of the primary circuit of the welding transformer in the welding machine which, as mentioned above, is a measure of the dynamic resistance of the weld being formed, and the system generates digital signals representative of the dynamic resistance of the weld which are appropriate for controlling the controller.
As is well known, resistance welding involves the localized melting and coalescence of a small volume of material due to the heating caused by the passage of electric current through the material. This heating is a function of the resistance of the material welded at the location of the weld. During the welding process, however, the resistance of the material being welded continually changes as the material is heated and melts. Until recently, weld characteristics have been determined by measuring the initial or static resistance at the beginning of the welding cycle, or the average current flowing during the welding operation. However, in order fully to characterize the welding process, a continuous or dynamic monitoring system is required which senses instantaneous changes in the weld resistance during the formation of the weld.
The present invention provides such a system, and one which generates digital signals representing the dynamic resistance of the weld being formed, which signals may be used to provide an automatic control for the controller, or to initiate appropriate indications to enable necessary manual controls to be made on the controller.
It is, accordingly, an objective of the present invention to provide an improved and simplified system for sensing the dynamic resistance of a weld as it is being formed, so that the weld portion of the welding cycle may be automatically terminated when the desired weld is achieved, and so that other automatic controls may be effectuated by the controller during the welding cycle.